This application is based on an application No. 034966/2000 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a developing system for forming a full-color image which is employed in a full-color image-forming apparatus wherein an electrophotography and an electrostatic printing are utilized.
2. Description of the Related Art
With the spread of a full-color development, the images wherein a full-color part and a black letter part coexist are in a fair way to a mainstream of the documents employed in an office. In recent years, many digital full-color machines have been proposed. The mainstream of the full-color machines is a machine wherein a full-color image is formed by superposing each color image of four kinds of color images (cyan, magenta, yellow and black images). Under the present situation, a percentage of an image-forming apparatus (copying machine or printer) wherein a monochromic mode and a full-color mode are united has been increased with a speeding up, said apparatus being referred to as monochrome/full-color united apparatus hereinafter.
In the case of the monochrome/full-color united apparatus, the same black toner is employed in the monochromic mode and full-color mode. In general, the black toner is prepared in such a way that a characteristic glossiness of the full-color image appears so as to achieve an excellent color reproducibility when it is used for forming the full-color image. For this reason, even if any of the monochromic mode and full-color mode is employed, a glossiness of letter parts becomes too high, and it is very difficult to read said letter.
Furthermore, the monochrome/full-color united apparatus has a problem that a speed of image formation in the monochromic mode is low. In general, the same developer-supporting member is employed in all developing devices (cyan developing device, magenta developing device, yellow developing device and black developing device). In the case where a speed of the developing system is increased, there is a problem that an unevenness of image concentration occurs when a black solid image is formed, since a followability (conveyability) of the black toner becomes worse, and the black toner cannot smoothly be conveyed.
On the other hand, two methods are known as a developing method. One is a two-component developing method wherein toner particles are electrified by friction between carrier particles and the toner particles. The others is a one-component developing method wherein the toner particles are electrified by passing through a clearance between the developer-supporting member and the developer-regulating member. With a digitization of the image-forming apparatuses, an opportunity to set the image-forming apparatuses by user""s side (e.g. in the immediate vicinity of a personal computer) has been increased. From a viewpoint of a saving of spaces in offices, the one-component developing method are widely employed, since said method can be conducted by means of a miniaturized apparatus.
To say more precisely, one-component developer is passed through the clearance between the developer-supporting member and the developer-regulating member which is arranged in contact with said developer-supporting member to form a thin-layer of the toner on said developer-supporting member and to electrify the toner particles in one-component developing method, said electrified toner particles being conveyed to a developing region as they are in order to develop an electrostatic latent image formed on a supporting member for said latent image. However, in this one-component developing method wherein the toner particles are electrified by passing the developer through the clearance between the developer-supporting member and the developer-regulating member, the toner particles are subjected to a high stress at the time of passing through said clearance, and a problem has been arisen that a satisfactory developing cannot be carried out, said problem being attributable to the following phenomena: (i) The toner particles are adhered to the developer-regulating member to lower a function of said member to form the thin-layer of the toner, and a poor electrification of the toner is brought about. (ii) A filming caused by a fusion of the toner particles occurs on the developer-supporting member (e.g. sleeve and the like). (iii) A fogging occurs on a photosensitive member (supporting member for electrostatic latent image). In addition, when the image is repeatedly copied by means of the apparatus wherein the one-component developing method is adopted, the aforementioned problems become remarkable to cause another problem that it is difficult to keep a stable toner characteristic.
In order to solve these problems, a technique for sphering a toner shape has recently been developed. It is thinkable that by sphering the toner shape, a crushing degree of the toner is decreased to suppress an occurrence of components having smaller particle size, and a decrease of the filming on the sleeve and the like can be possible. To put it concretely, a wet preparation method of a spherical toner by a suspension polymerization or an emulsion polymerization (Japanese Patent Publication (KOKAI) No. 257857/1989) and a spherization technique of a toner by heat treatment of a pulverized toner (Japanese Patent Publication (KOKAI) Nos. 319037/1988 and 317928/1994) have been proposed.
However, the spherization of the toner shape brings about an unsatisfactory conveyance of the toner because a powder pressure of the toner is decreased at a contact area with the developer-regulating member and an insertion of the toner in the clearance between the developer-supporting member and the developer-regulating member becomes difficult. If the unsatisfactory conveyance of the toner occurs, an unevenness of image concentration will occur when the solid image is formed.
Although the present inventors have attempted to increase the conveyability of the toner by giving a roughness to the surface of the developer-supporting member, sufficient durability and electrification stability of the toner could not be obtained when the former spherized toner is employed. In particular, with an increase of a toner-conveying speed of the developer-supporting member to cope with a high-speed system, the toner filming on the surface of the developer-supporting member, the toner fogging on the photosensitive member and the toner adhesion to the developer-regulating member were remarkably occurred, and the durability and electrification stability of the toner were remarkably decreased.
The present invention has been made in view of the aforementioned situation. The object of the present invention is to provide a developing system for forming a full-color image, wherein (i) an easiness of reading the black letter part is kept by suppressing a glossiness of said letter part, (ii) a speed-up of the monochromic development and an excellent color reproducibility of the full-color development can be achieved, (iii) the occurrences of the toner filming on the developer-supporting member and the toner fogging on the photosensitive member can be prevented over a long period of time, (iv) the toner can satisfactorily be conveyed to the developing region, and (v) the toner adhesion to the developer-regulating member can be prevented.
The present invention relates to a developing system for forming a full-color image which comprises
(i) a first developing device which is equipped with a developer-supporting member which carries one-component developing including a black toner and a regulating member which forms a thin layer of the black toner on the developer-supporting member, said developer-supporting member having a mean crest distance (Sm) of 20-200 xcexcm on its surface and satisfying the following relation:
d50/Ra=0.5-30
xe2x80x83(wherein d50 is a weight-average particle size (xcexcm) of the black toner, and Ra is a surface roughness (xcexcm) of the developer-supporting member), and
(ii) a second developing device which is equipped with a developer-supporting member which carries one-component developer including a first color toner and a regulating member which forms a thin layer of the first color toner on the developer-supporting member, said developer-supporting member having a mean crest distance (Sm) of 20-200 xcexcm on its surface and satisfying the following relation:
d50/Ra=0.5-3.0
xe2x80x83(wherein d50 is a weight-average particle size (xcexcm) of the first color toner, and Ra is a surface roughness (xcexcm) of the developer-supporting member), wherein tan xcex4 (150xc2x0 C.) of the black toner is not more than 1.0, and a first ratio of tan xcex4 represented by (tan xcex4 (150xc2x0 C.) of the first color toner/tan xcex4 (150xc2x0 C.) of the black toner) satisfies the following relation:
1.1xe2x89xa6the first ratio of tan xcex4xe2x89xa61.35.